It is well known that there are three states of matter: solids, liquid and gases. There is, however, a special fourth state of matter referred to as the liquid crystals (LCs) or mesomorphic states, intermediate between the solids and liquids. In the LC state, the material possesses long-range orientational order of the constituent units (molecules or molecular aggregates) while the long-range positional order of these units is partially or completely lost. The intermediate character of order is responsible for high sensitivity of LCs to external factors, such as the presence of electromagnetic fields or interface with another medium and also for unique optical and structural properties used in a variety of applications, ranging from computer monitors and other types of visual display systems commonly referred to as liquid crystal displays or LCDs, to materials of superior tensile strength such as Kevlar. The development of new properties and improvement of previously known properties may expand the number of applications in which liquid crystal materials may be used. One of these properties is the alignment of liquid crystal material on a substrate.
LCs may be classified as thermotropic or lyotropic. Thermotropic LCs are orientationally ordered (or mesomorphic) within a specific temperature range. In contrast, lyotropic LC materials become mesomorphic when dissolved in a solvent (such as water) within an appropriate concentration range. The LC state occurs within an appropriate range of parameters such as temperature and concentration.
Lyotropic LCs are typically amphiphilic materials (surfactants) formed by molecules that have a polar (hydrophilic) head and a non-polar (hydrophobic) aliphatic tail. This dual character of the molecules leads to self-organization, for example, micelle formation, when they are dissolved in a solvent such as water or oil. When lyotropic LC molecules are in contact with a substrate, their amphiphilic nature generally results in a perpendicular orientation of the molecule with respect to the plane of the substrate. Either the polar head of the lyotropic LC is attracted to a polar substrate or the hydrophobic tail of a lyotropic LC molecule is attracted to a non-polar substrate. Either orientation results in a perpendicular alignment of the molecule with respect to the substrate. This perpendicular alignment means that the preferred orientation is the so-called homeotropic alignment, in which the optical axis (or director) is perpendicular to substrate. However, it can be difficult to align the surfactant-based lyotropic liquid crystal in a planar fashion where the director is in the plane of the solid substrate.
Lyotropic chromonic liquid crystals (LCLCs) differ in their structure from conventional lyotropic LCs. Conventional lyotropic liquid crystals, also referred to as surfactants, are based on amphiphilic rod-like molecules with polar heads and hydrophobic alkyl chain tails. The term chromonic is a short hand expression for phrases such as “lyotropic mesophase formed by soluble aromatic mesogens.” The term was additionally intended to carry connotations of dyes and chromosomes and of the bis-chromone structure of disodium cromoglycate (DSCG), also known as cromolyn, one of the first identified solutes that form LCLCs. The molecular and macrostructure of LCLCs, as shown in FIG. 1, are generally plank-like or disk-like rather than rod-like, rigid rather than flexible, and aromatic rather than aliphatic. The LCLC molecules 10 (FIG. 1A) have a relatively rigid plank-like or disc-like aromatic core 12 with polar solubilizing groups 14 at the periphery. Aggregation of these molecules, caused by face-to-face arrangement of aromatic cores, results usually in cylindrical stacks 16 with molecular planes being more or less perpendicular to the axis of the aggregate. The geometry of the basic structural unit in LCLCs is thus very different from the micelles, spherical or cylindrical, and bilayers formed by amphiphilic (surfactant) molecules in the regular lyotropic liquid crystals. The tendency to aggregate is observed even in very dilute solutions, thus LCLCs do not show a distinct threshold concentration similar to the critical micelle concentration in amphiphilic systems, except, perhaps, at the stage of dimer formation. In contrast to the closed micelles formed by many surfactants, the LCLC aggregates do not have a clearly defined size (length), as there is no geometric restriction to the addition of another molecule to the existing stack. Such a behavior, first observed in the studies of nucleic acid bases and nucleosides, is called “isodesmic.”
The hydrophobicity of the aromatic core of the LCLC plays a role in the formation of rod-like aggregates. Hydrophilic ionic groups at the periphery of the molecules make the material water-soluble (see FIG. 1). The aggregates in water solution tend to be parallel to each other and often form the nematic type of liquid crystalline state, labeled N, in which the axes of rods are on average aligned along the same direction called the director, as shown in FIG. 1B.
Cromolyn, with the structural formula shown in FIG. 1A, has been used as an effective anti-asthmatic drug, although there is no clear link between its therapeutical activity and mesomorphic properties. When dissolved in water, cromolyn forms two basic mesomorphic phases, labeled N and M. The intermolecular separation is about 0.34 nm along the axis of aggregate, but it is believed that in the M phase, the aggregates are parallel to each other and arrange into a hexagonal lattice. In the N phase, formed at larger dilutions (shown in FIG. 1B), this lattice disappears but the aggregates retain an orientational order, which makes the N phase similar to a regular nematic phase in thermotropic (solvent-free) materials composed of elongated rod-like molecules.
The details of molecular packing within the LCLC aggregates, including cromolyn, are still debated. (see, for example, G. J. T. Tiddy, D. L. Mateer, A. P. Ormerod, W. J. Harrison, and D. J. Erdwards, Langmuir 11, 390 1995); P. Camorani, M. Furier, O. Kachkovskii, Yu. Piryatinsky, Yu. Slominskii, and V. Nazarenko, Semiconductor Physics, Quantum Electronics & Optoelectronics 4, 229 (2001).) Originally, it was assumed that in cromolyn aggregates, there is one molecule per circular cross-section of the cylinder; the cylinder diameter was estimated to be about 1.6 nm; this value is somewhat smaller than the extended length of the flat cromolyn molecule (which is about 1.9 nm, see FIG. 1). Later, it was proposed that the aggregate is of a hollow square-shaped chimney shape formed by four molecules linked by electrostatic salt bridges. It was further argued that the structure is close to that of regular micelles. Namely, there are four cromolyn molecules in the cross section of the aggregate; these molecules have a shape of a sharp letter V with the OH group at the apex. The OH groups point towards the axis of the cylinder while the eight zwitterionic COONa groups point outward to the water continuum.
Applications have taken advantage of the fact that the structure of LCLC materials in the LC state is orientationally ordered. As the result, the LCLC materials exhibit useful structural and optical properties, such as birefringence, polarization ability, polarization-dependent absorption and polarization-dependent light transmittance, ability to align other materials (such as thermotropic LCs). In most cases, the LC state is used to prepare the LCLC films with uniform alignment through some deposition technique that implies shear. Shear induces specific alignment of the director and thus the structural units (molecules or their aggregates) with respect to the shear direction. Once the aligned structure is created by shear in the LC state, it can be “frozen” by evaporating the solvent. It is important that the shear-induced alignment is especially pronounced when the LCLC material is in the LC state. If the material is too diluted so that it is in the isotropic fluid state during the shear, the alignment is often lost. However, it is important to realize that the final (“dried”) state of the material is not necessarily the state that satisfies the thermodynamic definition of the liquid crystalline or mesomorphic state.
In applications as mentioned above, such as polarizing, imaging and optical elements, there has been use of the dried-down films that are not necessarily in their liquid crystalline “mesomorphic” state anymore. Upon drying, the LCLC might display a variety of behaviors, as they might (a) crystallize; (b) form an amorphous solid in which the orientational order is largely preserved or “frozen”, the material, however, cannot easily flow; (c) remain in the LC state, either the same state (that was originally prepared by dissolving the LCLC material in solvent) or a different state that exists at lower concentrations of solvent. For example, in case of DSCG, the nematic N phase might transform into the columnar M phase and then into a crystalline state upon drying. Therefore, reference to “evaporated,” “dried-down”, or “dried” coating refers to coatings prepared from the LC state of LCLC materials in which the excess water (or other solvent) has been allowed to evaporate or has been removed by drying process, but which still retain an equilibrium moisture level typical of the conditions surrounding the sample.
The main feature allowing many LCLC applications in “dried” form is that when water evaporates, the resulting dry film still preserves an orientational order and thus anisotropic optical properties. Preserved in-plane long-range orientational order has been demonstrated not only for films of micron thickness but also for nanofilms comprised of just one or few stacked LCLC monolayers and fabricated by an electrostatic layer-by-layer deposition technique.
In practical applications of LCLCs, there is a need for a uniform alignment of LCLC materials and the dried films formed from LCLC when water (or other solvent) is evaporated, with the director in the plane of the cell or slightly tilted relative to the plane of the cell. One of the biggest problems in using LCLCs in making such films is that the LCLC director might develop a periodic pattern of distortions, including director variations in the form of sinusoidal lines, that are often called “tiger stripes,” “tiger texture” or ‘banded’ textures because of their characteristic textures under the polarizing microscope between crossed polarizers, as shown in FIG. 2A Because of the director deviation from a unidirectional texture, the optical properties of the resulting dry films are poor.
The director orientation at the substrate may be characterized with reference to two angles. The angle “theta” is the angle between the normal to the substrate and the director. The angle “alpha” is the angle between the director projection onto the substrate and the fixed axis x in the substrate. It has been documented that one of the physical mechanisms of the appearance of “tiger stripes” in nematic liquid crystals is the difference in polar angle “theta” that the director makes with the normal to the top surface of the LC film and the normal to the bottom surface of the LC film. The “tiger stripes” can, in principle, be characterized by modulation of either or both of these angles. However, even when “theta” is constant, for example, “theta”=Pi/2, the stripes can still exist because of the spatial variations of the angle “alpha” characterizing the director distortions in the plane of the substrate. In many applications, it would be desirable that the LCLC, and the dried version of it, align with the angles “alpha” and “theta” being substantially constant across the sample.
Methods for the alignment of thermotropic LCs are known in the art. For example, an attempt uses a substrate coated with a polymer with the polymer layer oriented, for example, by mechanical rubbing. The oriented film then provides an orientation direction for an overlaying thermotropic liquid crystal layer. However, such alignment techniques are not necessarily applicable to lyotropic LCs because of the structural differences between thermotropic and lyotropic LCs. A method of alignment of LCLCs by an aligned polymer layer or by a vapor deposited layer of silicon oxide or a similar compound on a substrate, has been developed, wherein the technique is applicable when the LCLC remains in its dissolved form, i.e. the solvent (water) remains within the sample. It would be desirable to provide alignment of LCLC material in its dried form.
Previously, additives used in the production of thin dried films of the LCLC materials were surfactants, such as Triton X-100, that improve wetting of the LCLC solution on the substrate, salts, such as NaCl, that change the phase diagram of the mesomorphic state of LCLC materials, pH-adjusting additives such as ammonium hydroxide, dyes to change the spectral characteristics of the films, pretilt agents such as Glucopon 225 (available from Henkel Corporation) to provoke a suitable orientation of an adjacent liquid crystal, cross-linking agents to improve chemical resistance of the chromonic materials, and 4-(dimethylamino) pyridine (DMAP), which improves the optical clarity of the liquid crystalline material. However, no previously used dopant has provided a uniform director orientation, and the use of different additives or dopants complicate predictions of chemical and physical properties of the LCLC materials.
Other techniques have also been suggested to impart a desired alignment to LCLCs. For example, it has been suggested to add a non-ionic surfactant to an LCLC, which can then be aligned by photo-treating an azobenzene-containing polymer to align the mixture of surfactant and LCLC. It is also known to align LCLCs in bulk solutions using a strong magnetic field applied to the LCLC cell. However, this field-induced alignment is only temporary as the degenerate (no fixed direction of molecular orientation) orientation returns within tens of minutes once the magnetic field is removed. Additionally, alignment of LCLC material may be accomplished by depositing alternating layers of polyion and LCLC on a substrate.
Aligned LC material may be used for the creation of polarizing films in liquid crystal displays (LCDs). One of the known techniques of preparing polarizing films is by using water-soluble dye solutions that form LCLCs on glass or polymer substrates. These films are visible to the human eye as they absorb in the visible part of the electromagnetic spectrum. The dyes form liquid crystal phases and tend to orient uniformly during the deposition of the solution on the substrate. However, closer inspection reveals numerous defects in the texture of the resulting film, such as the tiger stripes described above.
There is, therefore, a need to improve the alignment of the LCLC materials, to substantially eliminate the defects such as periodic variations in director orientation and thus to provide for alignment films with improved optical properties, specifically, LCLC-based alignment films that have a more uniform alignment.